Total energy expenditure can be measured using a double-labelled water technique. Water containing the stable isotopes 2 H and 18 O is given orally. As energy is expended carbon dioxide and water are produced. The difference between the rates of loss of the two isotopes is used to calculate the carbon dioxide production, which is then used to calculate energy expenditure. This can be done on urine samples over a 2- to 3-week period with the subject ambulatory. The technique is accurate, but it is expensive and requires the availability of a mass spectrometer. An alternative tracer technique for measuring total energy expenditure is to estimate CO2 production by isotopic dilution. A subcutaneous infusion of labelled bicarbonate is administered continuously by minipump and urine is collected to measure isotopic dilution by urea, which is formed from CO2 . Other methods for estimating energy expenditure, such as heart rate monitors or activity monitors, are also available but are less accurate.